The present invention is a method for removing metal contaminates from solution using mercapto-functional silica xerogels. The mercapto-functional silica xerogels are characterized by having a surface area greater than 365 m.sup.2 /g, an average pore diameter of at least 6 nm, and a pore volume of at least 2 cm.sup.3 /g. The mercapto-functional silica xerogels are particularly useful for removing Group VIII metals such as platinum from solutions comprising polyorganosiloxane fluids and resins.
Lentz, U.S. Pat. No. 3,122,520, teaches a method for making hydrophobic silica xerogels useful as fillers in silicone rubbers. The method comprises heating a silica hydrosol under strong acid conditions to form a hydrophilic silica hydrogel. The hydrophilic silica hydrogel is then contacted with a silane or siloxane treating agent to make a hydrophobic silica hydrogel. The hydrophobic silica hydrogel is reported to have a surface area in the dry state of from 100 to 650 m.sup.2 /g as determined by the BET method.
Burns et al., U.S. Pat. No. 5,750,610, extends the teachings of Lentz to a method useful for making mercapto-functional silica xerogels under acidic conditions.
Burns et al., U.S. Pat. No. 5,708,069, teach a method for making mercapto-functional silica xerogels under neutral conditions. The method taught by Burn et al. is particularly useful for making mercapto-functional silica xerogels useful in the present method. Burns et al. do not teach the present method for removing metal contaminants.
Feng et al., Science, Vol. 276, p. 923-925, May 9, 1997, describe treating the surface of a mesoporous silica material with tris(methoxy)mercaptopropylsilane. Feng et al. teach the treated silica is useful to remove heavy metals such as mercury, lead, and silver from aqueous and organic solutions.
Howard et al., Analyst, Vol. 112, p. 159-162, October 1987, teach a silica gel surface modified with (3-mercaptopropyl)trimethoxysilane useful in selectively removing arsenite from aqueous solution. Howard et al. provide no physical description of the silica gel.
Volkan et al., Anaylst, Vol. 112, p. 1409-1412, October 1987, teach the surface modification of a silica gel with (3-mercaptopropyl)trimethoxysilane. Volkan et al. describe the silica gel as "Kieselgel 60, 230-400 mesh," with no further description as to manufacturer or physical characteristics. Volkan et al. teach the modified silica gel can be used to remove cadmium, copper, lead, and zinc from aqueous solution.
Filho et al., Separation Science and Technology, Vol. 32, No. 15, p. 2535-2545, 1997, teach a silica gel surface modified with 2-mercaptoimidazole having a surface area of 365 m.sup.2 /g and an average pore diameter of 6 nm. Filho et al. teach this surface modified silica is especially useful for removing mercury from aqueous solutions.